Electrochemical cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages, including improved safety features. Notwithstanding their advantages, the use of certain of these solid batteries over repeated charge/discharge cycles may be substantially impaired when they exhibit significant drops in their charge and discharge capacity over repeated cycles as compared to their initial charge and discharge capacity.
Specifically, solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix as well as a suitable inorganic ion salt. The inorganic matrix may be non-polymeric [e.g., .beta.-alumina, silicon dioxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic constituents include, by way of example, polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrochemical cells containing a solid electrolyte, i.e. a polymeric matrix may suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain a solvent (plasticizer) which is typically added to the matrix in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrochemical cell.
Suitable solvents, well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
The solid, solvent-containing electrolyte is typically formed in one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (i.e., triglyme and the organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrochemical cell).
Notwithstanding the above, the initial capacity of solid batteries is often less than desirable. Moreover, even when the initial capacity of the solid battery is relatively high, such solid batteries often exhibit rapid decline in capacity over their cycle life.
Specifically, the cumulative capacity of a solid battery is the summation of the capacity of a solid battery over each cycle (charge and discharge) in a specified cycle life. Solid batteries having a high initial capacity but which rapidly lose capacity over the cycle life will have low cumulative capacity which interferes with the effectiveness of these batteries for repeated use.
The normal passivation layer of lithium surfaces in contact with the described electrolytes are relatively brittle and unstable. When the battery is cycled, the lithium is stripped from the anode on the discharge half-cycle, and is plated on the anode in the recharge half-cycle. The weak passivation layer is disrupted by the cyclic process and the lithium metal reacts further with the electrolyte. Consequently, the lithium is stripped and plated selectively over the anode surface, causing the growth of dendrites, or nodules, which can eventually short the cell through the electrolyte.
It would be advantageous if the lithium surface could be enhanced by a strong and stable passivation layer which is ionically conductive.